In silver halide photography, continuous tone images are traditionally formed by exposing a photographic element to an image and developing the element to form a corresponding image therein. In black and white photography, the element's image will usually be of a single record. In color photography, the element's corresponding image is usually made up of three records: a yellow record, a magenta record, and a cyan record, corresponding to the blue, green, and red portions of the original image.
Photographic elements are generally of two types: negative or reversal. For either of these types, it is often desirable to produce a negative or positive copy. This copying is done by exposing a second photographic element with light that is transmitted through a previously exposed and processed first element made on transparent base or with light that is reflected from a previously exposed and processed first element made on a reflective base. The second element is then developed to yield the copy.
The ability of a photographic element, such as the second photographic element described above, to reproduce the contrast, i.e., the range of image densities, of an image is usually determined by the slope of the straight-line portion of the characteristic curve, i.e., the D-Log E curve (a plot of image density versus log exposure). This slope is referred to as gamma and is a measure of the contrast characteristics of a photographic element. "Contrast" will be used herein to refer to the qualitative appearance of the image, as opposed to other usages in the art where contrast has sometimes been used interchangeably for gamma. Another way to quantify the contrast of an image, independent of gamma, is by the range of densities found in the image. A lower contrast image will typically have a lower image density range than a higher contrast image.
If it is desired to replicate the contrast of an image, a photographic element having a gamma with an absolute value of approximately unity is used. When it is desired to produce a photographic copy having a lower contrast than the original image, a photographic element having a gamma with an absolute value of less than 1 is used. Similarly, a photographic element having a gamma with an absolute value of greater than 1 is used to produce a photographic copy having greater contrast than the original. These three scenarios are illustrated in FIGS. 1-3, as described below.
FIGS. 1-3 are four-quadrant objective tone scale reproduction diagrams, similar to those shown in B. Carroll, G. Higgins, and T. James, Introduction to Photographic Theory, chapt. 5, Wiley Publ., New York, 1980. FIG. 1 represents the matched-contrast scenario, FIG. 2 represents the reduced-contrast scenario, and FIG. 3 represents the increased-contrast scenario. In each of these Figures, the image density range of the original is represented on the horizontal axis at the top of Quadrant 1 (Q1). The densities represented by this range are the input data for lines 11, 21, and 31 of FIGS. 1, 2, and 3, respectively. Line 11, 21, or 31 is a straight line having a slope of -1, performing the function of mapping the input data representing the densities of the original from Quadrant 1 into Quadrant 2 (Q2). Line 12, 22, or 32 in Quadrant 2 is a line having a slope of 1, representing the mapping of the density input from the original to a log exposure output that is provided to the photographic element onto which the copy is made. Curves 13, 23, and 33 in Quadrant 3 (Q3) represents the characteristic D-Log E curve of a negative-working photographic element onto which the copy is made. In the matched contrast scenario represented by FIG. 1, curve 13 has a straight-line slope (i.e., gamma) of 1. In the reduced-contrast scenario represented by FIG. 2, curve 23 has a straight-line slope (i.e., gamma) of less than 1. In the increased-contrast scenario represented by FIG. 3, curve 33 has a straight-line slope (i.e., gamma) of greater than 1. The input log exposure values are mapped through curves 13, 23, and 33 to give the densities of the final copy image on the vertical axis between Quadrants 3 and 4. These D-log E curves must have straight-line portions long enough to cover the density range of the original image. Line 14, 24, or 34 in Quadrant 4 (Q4) is a straight line having a slope of 1, which performs the function of mapping the densities of the copy image onto the horizontal axis at the top of Quadrant 1, so that the density range of the copy can be compared with the density range of the original. The above-described mapping operations are represented by dotted lines 15, 25, and 35, and dashed lines 16, 26, and 36. These lines map a representative low and a representative high density on the original, through Quadrants 1, 2, 3, and 4 in the direction of the arrows shown on lines 15, 25, 35, 16, 26, and 36, ending up as densities on the copy on the horizontal axis at the bottom of Quadrant 4. In the matched contrast scenario represented by FIG. 1, it is seen that the density range of the copy is the same as the density range of the original. In the reduced-contrast scenario represented by FIG. 2, it is seen that the density range of the copy is smaller than the density range of the original. In the increased-contrast scenario represented by FIG. 3, it is seen that the density range of the copy is greater than the density range of the original.
When it is desired to make a photographic copy of an original image having the same or reduced contrast as the original image, the photographic element onto which the copy is made traditionally must have a gamma with an absolute value of less than or equal to about 1. In order to achieve a satisfactory D-max in an element with a gamma of 1 or less, the emulsion system used in the element must have a broad exposure latitude. When relatively monodispersed emulsions are used, it is often necessary to use multiple emulsions having substantially the same spectral sensitivity but different speeds to achieve the needed latitude. This is illustrated in FIG. 4, where curve 100 represents a faster, short-latitude emulsion having larger grain sizes, curve 110 represents a slower, short-latitude emulsion having smaller grain sizes, curve 130 represents the additive latitude-broadening effect of the two emulsions. Curve 140 represents a single short-latitude emulsion that achieves the desired D-max, but which necessarily has a high gamma that would not produce a copy having a contrast that is the same as or lower than the original.
The necessity of multiple silver halide emulsions for each region of spectral sensitivity increases the complexity, difficulty of preparation, and expense of the photographic element, whether they are coated in separate layers or blended together in a single layer. Moreover, the presence of larger silver halide grains that are especially prevalent in the faster emulsions can lead to light scattering, which reduces the sharpness of the image produced in the element.
An alternate method for achieving the latitude needed to give a desired D-max with a relatively low gamma is to use a highly polydisperse emulsion. However, such highly polydisperse emulsions are difficult to chemically and spectrally sensitize in an optimum fashion, since each of the grain size classes within the emulsion is likely to require a different concentration of reagents to achieve this optimum sensitization. Consequently, the speed/fog characteristics of such emulsions are frequently inferior to monodisperse emulsions. In addition, reproducible precipitation of a highly polydisperse emulsion is often more difficult than reproducible precipitation of monodisperse emulsion. Further, the population of larger grains that are present in highly polydisperse emulsions will contribute to additional light scattering, again reducing the sharpness of the image produced in the element.
It would thus be desirable to produce color copies having the same as or lower contrast as an original image using a photographic element that does not require either multiple silver halide emulsions for each region of spectral sensitivity or highly polydisperse emulsions, and their associated disadvantages. As described above, such an element necessarily has a high gamma (over 1), which, using prior art processes, would produce an image having not the desired same or lower contrast, but an undesired greater contrast than the original image.